The recent discovery' of groundwater contaminated with carbamate pesticides has made it imperative to have reliable methods for detecting low levels (ppb') of these environmentally persistent pesticides in soil and water. One of the most toxic of these pesticides used on crops is aldicarb [2-methyl-2-(methylthio)propionaldehyde-0-(methylcarbamoyl) oxime]. Its residues primarily consist of the parent compound and two principal oxidation products, aldicarb sulfoxide and aldicarb sulfone. Extensive contamination of groundwater by aldicarb metabolites has been discovered in over one thousand wells on eastern Long Island'. This contamination resulted from the use of aldicarb on potato fields to protect the crop from the Colorado potato beetle and the golden nematode. The residues found in the groundwater consist on the average of equal parts of the sulfoxide and sulfone both of which are similar in toxicity to aldicarb.
Numerous methods for the determination of aldicarb and its oxidation products have been reported in the literature 2-6. Gas chromatography is often the method of choice for quantitative determination of residues in crops and more recently soil and water. Since aldicarb and aldicarb sulfoxide are not resolved from the solvent peak, the usual procedure is to oxidize these species to the sulfone with peracetic acid. After bulk extraction and cleanup, the total aldicarb residue can then be quantified by gas chromatography with a flame photometric detector2-4, an electrolytic conductivity detectors, or a nitrogen phosphorus detector'. If quantification of the individual metabolites is desired it is necessary to separate them by column chromatography after the extraction step. Aldicarb and aldicarb sulfoxide would then be oxidized separately to aldicarb sulfone before analysis. These procedures are both time consuming and contribute to inaccurate determinations.
High-performance liquid chromatography has also been used for quantification of low levels of aldicarb and its oxidation products in waters+. Analysis of the three species after bulk extraction was reported using a two injection procedure'. A method of direct analysis of aldicarb after a preconcentration step has also been developed6.
l Throughout this article the American billion (109) is meant.